Configurable tapered delay chain with multiple sizes of delay elements

ABSTRACT

A tapered chain of delay elements. The chain of delay elements includes a plurality of delay elements comprising a plurality of smaller sized stacked inverter delay elements each configured to implement a first delay, and a plurality of larger sized stacked inverter delay elements each configured to implement a second delay larger than the first delay. A switch circuit is coupled to the plurality of delay elements and is configured to select at least one of the plurality of delay elements to create a delay signal path having an amount of delay in accordance with a number of delay elements comprising the delay signal path. An input is coupled to a first delay element of the delay signal path to receive an input signal. An output is coupled to the switch circuit, wherein the output is coupled to the delay signal path to receive a delayed version of the input signal after propagating through the delay signal path.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of co-pending, commonly assigned U.S. patent application Ser. No. 10/864,271, filed Jun. 8, 2004 now U.S. Pat. No. 7,336,103, entitled “Stacked Inverter Delay Chain” to Masleid and Burr, which is hereby incorporated herein by reference in its entirety.

This Application is related to, and incorporates by reference in their entirety, the following co-pending, commonly assigned United States patent applications:

U.S. patent application Ser. No. 11/020,746, filed Dec. 23, 2004, entitled “A CONFIGURABLE TAPERED DELAY CHAIN WITH STACKED INVERTER DELAY ELEMENTS” by Masleid;

U.S. patent application Ser. No. 11/021,221, filed Dec. 23, 2004, entitled “A CONFIGURABLE DELAY CHAIN WITH SWITCHING CONTROL FOR TAIL DELAY ELEMENTS” by Masleid;

U.S. patent application Ser. No. 11/021,632, filed Dec. 23, 2004, entitled “POWER EFFICIENT MULTIPLEXER” by Masleid;

U.S. patent application Ser. No. 11/021,197, filed Dec. 23, 2004, entitled “LEAKAGE EFFICIENT ANTI-GLITCH FILTER WITH VARIABLE DELAY STAGES” by Masleid; and

U.S. patent application Ser. No. 11/021,633, filed Dec. 23, 2004, entitled “LEAKAGE EFFICIENT ANTI-GLITCH FILTER” by Masleid.

TECHNICAL FIELD

The present invention relates to signal timing for digital integrated circuit devices.

BACKGROUND ART

The design and fabrication of high-performance signaling mechanisms for digital integrated circuit devices has become a significant challenge. For example, with respect to high-performance memory integrated circuit devices, ensuring the reliability in the design and fabrication of the signaling components of such devices (e.g., high performance DDR memory) has become problematic. In the past, slower memory bus speeds allowed significant specification margins in the design and fabrication of a given memory module. However, modern memory integrated circuit designs require exacting control of critical timing specifications, and design parameters must be strictly maintained to keep the entire system in balance. A variable signal delay element is a mechanism used to compensate for timing irregularities and calibrate sensitive signaling components. What is needed is an effective variable delay element that provides reliability, speed, and high granularity timing to insure an overall system (e.g., CPU, bridge components, peripheral busses, etc.) operates at peak performance, while requiring minimal power.

DISCLOSURE OF THE INVENTION

Embodiments of the present invention provide a method and system for a configurable tapered delay chain with multiple sizes of leakage efficient delay elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:

FIG. 1 illustrates a schematic of a stacked inverter delay chain, in accordance with embodiments of the present invention.

FIG. 2 illustrates an exemplary layout of a stacked inverter, in accordance with embodiments of the present invention.

FIG. 3 illustrates a flow chart of steps in a method of delaying a signal, in accordance with embodiments of the present invention.

FIG. 4 shows a diagram depicting a stacked inverter delay chain in accordance with embodiments of the present invention.

FIG. 5 shows a configurable tapered delay chain in accordance with one embodiment of the present invention.

FIG. 6 shows a diagram depicting the internal components of the switch circuit in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of embodiments of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the embodiments of the present invention.

NOTATION AND NOMENCLATURE

Some portions of the detailed descriptions which follow are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. A procedure, computer executed step, logic block, process, etc., is here, and generally, conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present invention, discussions utilizing terms such as “storing” or “accessing” or “recognizing” or “retrieving” or “translating” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention implement a tapered chain of delay elements optimized for minimal leakage current. In one embodiment, the chain of delay elements includes a plurality of delay elements comprising a plurality of smaller sized stacked inverter delay elements each configured to implement a first delay, and a plurality of larger sized stacked inverter delay elements each configured to implement a second delay larger than the first delay. A switch circuit is coupled to the plurality of delay elements and is configured to select at least one of the plurality of delay elements to create a delay signal path having an amount of delay in accordance with a number of delay elements comprising the delay signal path. An input is coupled to a first delay element of the delay signal path to receive an input signal. An output is coupled to the switch circuit, wherein the output is coupled to the delay signal path to receive a delayed version of the input signal after propagating through the delay signal path. Additional descriptions of embodiments of the present invention follow below.

The following description of embodiments in accordance with the present invention is directed toward delay elements having PFETs (or p-type field effect transistors formed in surface N-wells and/or NFETs (or n-type field effect transistors) formed in surface P-wells when a p-type substrate and an N-well process are utilized. It is to be appreciated, however, that embodiments in accordance with the present invention are equally applicable to NFETs formed in surface P-wells and/or PFETs formed in surface N-wells when an n-type substrate and a P-well process are utilized. Consequently, embodiments in accordance with the present invention are well suited to semiconductors formed in both p-type and n-type materials, and such embodiments are considered within the scope of the present invention.

FIG. 1 illustrates a schematic of a novel stacked inverter delay chain 100, in accordance with embodiments of the present invention. Stacked inverter delay chain 100 comprises stacked inverters 110 and 120. The output of stacked inverter 110 is coupled to the input of stacked inverter 120. It is to be appreciated that additional stacked inverter delay chains, e.g., one or more instances of stacked inverter delay chain 100, can be coupled to stacked inverter delay chain 100 to achieve larger signal delay values. Examples of multiple stacked inverter delay chains arranged a large configurable stacked inverter delay system in accordance with embodiments of the present invention are described in the discussion of FIG. 4 below.

In contrast to a conventional inverter, stacked inverters 110 and 120 comprise more than a single p-type device coupled to a single n-type device. Rather, stacked inverters 110 and 120 comprise multiple p-type devices and multiple n-type devices. More particularly, stacked inverter 120 comprises two p-type devices 121 and 122 and three n-type devices 123, 124 and 125. The gates of the devices of stacked inverter 120 are coupled together forming the input of the inverter stage. The output of the inverter stage may be taken at the coupling of a p-type device to an n-type device.

In contrast to a conventional inverter, however, stacked inverter 120 comprises multiple series devices per “leg.” For example, two p-type devices are configured to pull the output high (when appropriate) and three n-type devices are configured to pull the output low. Consequently, the drive capability of stacked inverter 120 is less than the drive capability of a conventional inverter. Beneficially, such decreased drive capability produces an increased delay of a signal through stacked inverter 120.

Additionally, and also of benefit, stacked inverter 120 presents an increased load to its driving circuitry, in comparison to a conventional inverter. For example, a signal input to stacked inverter 120 is coupled to five active devices as opposed to being coupled to two active devices in a conventional inverter. Each device presents an input capacitance. Such increased loading produces a further desirable increase in signal propagation delay.

An approximate analysis of stacked inverter delay chain 100 indicates that the delay of stacked inverter 120 is about six times the delay of a conventional two-device inverter. For example, drive resistance of stacked inverter 120 can be about 2.5 times the drive resistance of a conventional inverter, and load capacitance of stacked inverter 120 can be about 2.5 times the load capacitance of a conventional inverter. If stacked inverter 110 is constructed similarly, the delay through stacked inverter delay chain 100 will be about 6 times longer than through a conventional inverter pair. In different terms, a delay through stacked inverter delay chain 100 is approximately the same as a delay through a chain of 12 stages of conventional inverters. It is appreciated that an exact evaluation is the province of circuit simulation and the details of a particular semiconductor manufacturing process.

A chain of 12 conventional inverters comprising 24 active devices has approximately the same delay as stacked inverter delay chain 100 comprising ten active devices. Consequently, the active switching power of stacked inverter delay chain 100 is beneficially reduced to approximately 42 percent (10 divided by 24) of the active switching power of a conventional delay circuit, for about the same delay.

In addition to a reduction in the number of active devices required for a comparable delay, a beneficial reduction is realized in integrated circuit die area required by stacked inverter delay chain 100. As a consequence of utilizing fewer active components than a conventional delay circuit, stacked inverter delay chain 100 comprises about 42 percent of the die area for active devices versus a conventional delay circuit, for approximately the same. However, there is yet another additional integrated circuit die area benefit realized by stacked inverter delay chain 100 over the conventional art.

FIG. 2 illustrates an exemplary layout of stacked inverter 120, in accordance with embodiments of the present invention. It is appreciated that FIG. 2 is not drawn to scale.

Stacked inverter 120 comprises two p-type devices (121, 122 of FIG. 1) formed in p-type diffusion within n-well 220. Stacked inverter 120 comprises three n-type devices (123, 124, 125 of FIG. 1) formed in n-type diffusion. Metallization 240 couples p-type diffusion 220 with n-type diffusion 235, coupling p-type device 122 (FIG. 1) with n-type device 123 (FIG. 1) and forming the output of stacked inverter 120 (FIG. 1).

Metallization 260 couples p-type device 121 (FIG. 1) to an operating voltage, e.g., Vdd. Metallization 250 couples n-type device 125 (FIG. 1) to ground. Metallization 270 couples an input signal to the gates of all devices.

In a conventional art delay circuit, all diffusion regions require a contact. For example, in the conventional art, contacts are required to connect a transistor to a later stage and/or to connect a transistor to a transistor of opposite type. Thus, a conventional art inverter chain requires about 5 contacts per stage. For example, a conventional art inverter stage would typically comprise one contact to couple Vdd to the p-type device, one contact to couple ground to the n-type device, one contact to couple the inverter output to the p-type device, one to couple the inverter output to the n-type device and one contact for the input. Twelve stages of inverters thus require about 60 contacts. It is appreciated that additional contacts are generally required for coupling Vdd and ground to the wells.

In contrast, in accordance with embodiments of the present invention, stacked inverter delay chain 100 requires far fewer contacts to produce about the same delay as a conventional 12-stage inverter delay chain. In contrast to the conventional art, stacked inverter 120 has no need of contacts within its stacks. For example, no contact is necessary between p-type devices 121 and 122 (FIG. 1), nor is a contact necessary between n-type devices 123 and 124 (FIG. 1), nor is a contact necessary between n-type devices 124 and 125 (FIG. 1). For example, one contact couples p-type device 121 (FIG. 1) to Vdd (contact 209), and one contact couples n-type device 125 (FIG. 1) to ground (contact 210). One contact couples p-type device 122 (FIG. 1) to the output (contact 211), and one contact couples n-type device 123 (FIG. 1) to the output (contact 212). One contact couples the input to all devices (contact 213). Consequently, stacked inverter 120 can be constructed utilizing a total of about five contacts. Exemplary contacts 201-208 are illustrated coupling Vdd and ground to the wells. It is appreciated that such contacts are commonly interspersed at intervals, e.g., every tenth row of logic, and thus may not be strongly associated with a particular circuit.

Therefore, in contrast to a conventional art inverter delay chain requiring about 60 contacts, stacked inverter delay chain 100 requires only about 10 contacts, or one sixth as many contacts to produce about the same delay. Consequently, embodiments in accordance with the present invention yield highly advantageous integrated circuit die area reductions far beyond a reduction in transistor count.

A further benefit of stacked inverter delay chain 100 derives from utilizing fewer stages in comparison to the conventional art. Consequently, embodiments in accordance with the present invention require less wiring to intercouple stages and fewer inter-stage spaces to separate stages. Such requirements for less wiring and less space result in a desirable reduction in integrated circuit die area required for such wiring and spaces.

It is to be appreciated that static power consumption in modern semiconductor processes, e.g., processes with a minimum feature size of about 0.13 microns and smaller, is no longer a negligible component of total power consumption. For such processes, static power may be one-half of total power consumption. Further, static power, as a percentage of total power, is tending to increase with successive generations of semiconductor process.

Embodiments in accordance with the present invention offer significant advantages in reducing static power consumption in comparison with the conventional art. A conventional art inverter delay chain comprises a leakage path for each inverter, e.g., a series “string” of devices from operating voltage (Vdd) to ground. Thus, a 12 inverter delay chain comprises 12 leakage paths. In contrast, stacked inverter delay chain 100 comprises just two leakage paths. Consequently, stacked inverter delay chain 100 comprises one sixth of the leakage paths.

Further, such leakage paths within stacked inverter delay chain 100 suffer less leakage than conventional inverters, yielding additional beneficial leakage reductions. In a conventional inverter, exactly one transistor is on while the other transistor is off. As an unfortunate consequence, approximately the full bias voltage is applied to the off transistor, resulting in a maximum possible leakage for the off transistor.

In contrast, referring once again to FIG. 1, in stacked inverter 120 multiple transistors are either on or off in series. For example, for a “high” output state, transistors 121 and 122 are on, while transistors 123, 124 and 125 are off. Consequently, each off transistor (123-125) has significantly less than full bias voltage applied. For example, for a high output, each transistor 123, 124 and 125 will have about one third of full bias voltage applied. It is appreciated that leakage current generally decreases exponentially as voltage decreases. For example, a two times reduction in off bias voltage produces about an eight times reduction in leakage current per leakage path.

It is to be further appreciated that such leakage induces non zero voltages at intermediate nodes between the off transistors, e.g., between transistors 125 and 124, and between transistors 124 and 123. Such voltages induce body effects in the transistors. Such body effects increase the threshold voltage of the affected transistors. An increased threshold voltage generally produces beneficial decreases in leakage current.

Consequently, in addition to a decrease in a number of leakage paths, in accordance with embodiments of the present invention, the leakage current of each path is very beneficially reduced due to an induced body effect and a highly non-linear relationship between bias voltage and leakage current. An approximate analysis indicates that total leakage current of stacked inverter delay chain 100 is reduced about 50 times in comparison to a conventional delay chain of inverters, for the same delay.

Another aspect of merit regarding delay circuits is the ability of a delay circuit to track speed changes of other circuitry of an integrated circuit. It is appreciated that a variety of factors, e.g., operating voltage, operating temperature and/or manufacturing process variations, can affect the speed of operation of an integrated circuit. It is generally desirable for a delay circuit to track speed changes of other circuitry of an integrated circuit. For example, if other circuits of an integrated circuit operate faster, generally less absolute delay is required from a delay circuit for the overall circuit to function. Because embodiments in accordance with the present invention comprise stacked devices, they are similar to many logic circuits that also comprise stacked devices, e.g., NAND and/or NOR logic gates. Consequently, embodiments in accordance with the present invention match or track changes in operating speed of complex logic more accurately than delay chains comprising very simple inverters.

Embodiments in accordance with the present invention are thus shown to offer significant and highly beneficial improvements in tracking timing changes of other circuits, integrated circuit die area, active power consumption and static power (leakage current) consumption in comparison to the conventional art.

FIG. 3 illustrates a flow chart of steps in a method of delaying a signal 300, in accordance with embodiments of the present invention. In block 310, the signal is inverted using a first stacked inverter circuit to produce an inverted signal at an output of the first stacked inverter circuit. For example, the inverted signal is the output of stacked inverter chain 110 of FIG. 1.

In block 320, the inverted signal is propagated to an input of a second stacked inverter circuit, e.g., at the input of stacked inverter chain 120 of FIG. 1. In block 330, a delayed version of the signal is produced at an output of the second stacked inverter circuit. For example, in reference to FIG. 1, a delayed version of the input to stacked inverter circuit 110 is produced at the output of stacked inverter chain 110. In accordance with embodiments of the present invention, the first and the second stacked inverter circuits comprise at least five active devices.

It is to be appreciated physical differences between electrons and holes, and between n-type and p-type dopants, as well as constructive differences in device geometry and dopant placement, result in differences in efficiency between n-type devices and p-type devices. Because electron mobility is higher than hole mobility, n-type devices are more efficient than p-type devices. However, the degree of difference depends on constructive differences that can vary with process. Such physical and constructive differences also produce other behavior differences, such as a difference in sensitivity to body effects. Consequently, different levels of benefit, e.g., in leakage reduction, are to be expected between stacks of n-type devices and stacks of p-type devices. To allow for such effects, in accordance with embodiments of the present invention, it is possible to stack different numbers of transistors on either or both legs of a stacked inverter. Such variations allow increases in load and/or decreases in drive capability, enabling a wide variety of delay values, as well as enabling differing body effects.

For example, depending upon a wide variety of factors, including, e.g., details of a semiconductor process, required delay, active power budget and/or static power budget, a delay circuit comprising multiple stacked inverter circuits, each comprising three or more p-type devices in conjunction with three or more n-type devices, may better optimize available resources than stacked inverter delay circuit 100 (FIG. 1).

It is to be appreciated that conventional integrated circuit design practice generally teaches away from embodiments in accordance with the present invention. For example, much of the art generally teaches design of “fast” circuits. In most areas of integrated circuit design, a great deal of effort is devoted to design details that contribute to an increased speed (frequency) of operation, e.g., reducing input capacitance and increasing output drive. For example, in contrast to conventional teaching and practice, stacked inverter chain 120 comprises stacked transistors without an intermediate buffer, reducing output drive capability and slowing the circuit down. Further, stacked inverter chain 120 comprises multiple inputs that all have the same logical purpose, increasing input capacitance and further slowing the circuit down.

Further, embodiments in accordance with the present invention are contrary to the operation of conventional integrated circuit design tools. For example, conventional design synthesis tools will routinely “optimize” redundancy out of a design. For example, stacked inverter 120 (FIG. 1) comprises two field effect transistors in series driven by the same input. From a logic design perspective, such a structure may be considered redundant. Thus, conventional design synthesis tools will routinely reduce stacked inverter 120 to a conventional two-device inverter. Consequently, a designer may be required to take custom efforts to retain and embody a novel stacked inverter in accordance with embodiments of the present invention when utilizing conventional design tools.

Embodiments in accordance with the present invention provide a stacked inverter comprising desirable delay, die area and power characteristics. Further embodiments in accordance with the present invention provide for coupling two stacked inverters together to form a stacked inverter delay chain that is more efficient in terms of die area, active and passive power consumption than conventional delay chains comprising conventional inverters. Still further embodiments in accordance with the present invention provide for stacks of varying numbers of devices per leg of a stacked inverter.

Additional descriptions of stacked inverter delay elements can be found in commonly assigned United States patent application “STACKED INVERTER DELAY CHAIN” by Masleid et al., filed on Jun. 8, 2004, application Ser. No. 10/864,271, which is incorporated herein in its entirety.

FIG. 4 shows a diagram depicting a stacked inverter delay chain 400 in accordance with embodiments of the present invention. The stacked inverter delay chain 400, hereafter delay element 400, comprises four stacked inverters 410-440. The output of stacked inverter (e.g., stacked inverter 410) is coupled to the input of next stacked inverter (e.g., stacked inverter 420) of the delay element. In this manner, the delay element 400 implements a larger amount of delay a comparison to the smaller delay element 100 shown in FIG. 1.

As described above, it should be noted that different numbers of stacked inverters (e.g., one or more instances of stacked inverter 410), can be coupled per delay element to achieve differing amounts of signal delay values. For example, delay elements that achieve large amounts of signal propagation delay would incorporate a larger number of stacked inverters, and so on. In addition to changing the number of stacked inverters per delay element, the amount of delay implemented by a given delay element can be tailored by changing the number of devices comprising each stacked inverter of the delay element (e.g., from two devices, to four devices, six devices, etc. per inverter). Additionally, it should be noted that depending upon the number of stacked inverters per delay element, the signal emerging from one delay element to the next delay element will be inverted (e.g., for an odd number of inverter(s) per delay element) or un-inverted (e.g., for an even number of inverters per delay element), and this signal attribute needs to be appropriately handled in the output circuitry (e.g., the switch circuit 540 as shown in FIG. 6 below).

Referring now to FIG. 5, a configurable tapered delay chain 500 in accordance with one embodiment of the present invention is shown. As depicted in FIG. 5, the delay chain 500 includes a plurality of delay elements, comprising a first group of small delay elements 501, a second group of medium delay elements 502, and a third group of large delay elements 503. In the present embodiment, the delay element comprising the delay chain 500 are stacked inverter delay chains having a configuration substantially similar to the stacked inverter delay chain 100 of FIG. 1, the stacked inverter delay chain 400 FIG. 4, and so on.

In the present embodiment, the small delay elements 501 are configured to implement a comparatively small amount of delay. Similarly, the medium delay elements 502 are configured to implement a larger delay in comparison to the elements 501, and the large delay elements 503 are configured to implement an even larger delay in comparison to the elements 502. The differing amounts of delay are implemented by using different numbers of stacked inverters (e.g., stacked inverter 110) per delay element, in the manner described above.

In the present embodiment, each of the delay elements 501-503 are coupled in series to one another, with the input of each subsequent delay element directly coupled to the output of the immediate prior delay element. This is shown by the arrows 505 and 506. Each of the delay elements 501-503 are coupled to a switch circuit 540. The switch circuit 540 includes an output 520 for providing the resulting output signal 520 to, for example, other external circuits.

In the present embodiment, each of the delay elements 501-503 comprises a leakage efficient stacked inverter delay chain of the configuration described above (e.g., stacked inverter chain 100). Accordingly, the configurable stacked inverter delay chain 500 provides substantial benefits with regard to tracking timing changes of other circuits, integrated circuit die area, active power consumption and static power (e.g., leakage current) consumption in comparison to the conventional art.

The switch circuit 540 is coupled to the each of the delay elements 501-503 and is configured to select at least one of the plurality of delay elements 501-503 to create a delay signal path having an amount of delay in accordance with a number of stacked inverter delay elements comprising the delay signal path, and the type of inverter delay element comprising the delay signal path (e.g., small, medium, large, etc.).

For example, to implement a resulting output signal 520 having the smallest amount of delay, the switch circuit 540 would implement a delay signal path including only the first delay element of the group of small delay elements 501. The input signal 510 would propagate to the first delay element of group 501 and incur an amount of delay in accordance with the devices of the first delay element. The signal emerging from the first delay element of group 501 is selected by a tap (e.g., tap 521) of the switch circuit 540. The signal is coupled to the output 520 by the switch circuit 540 as the resulting delayed output signal 520.

To implement a resulting output signal 520 having a larger amount of delay, the switch circuit 540 implements a delay signal path including an additional number of the delay elements. For example, the amount of delay added to the input signal 510 can be substantially doubled by selecting tap 522 of the switch circuit. This causes the input signal 510 to propagate through the first delay element of group 501 through the next delay element of group 501 before being picked up by the tap 522. Thus, the resulting output signal 520 will have an added amount of delay including the delay from two small delay elements of group 501.

In this manner, the switch circuit 540 is configured to implement a configurable, adjustable delay signal path by selecting the appropriate one of its taps. The switch circuit 540 adjusts the delay signal path by switching delay elements into or out of the delay signal path via one of its taps. In the present embodiment, the output of the prior delay element is coupled to the input of a subsequent delay element via a substantially direct connection.

In the present embodiment, the differing sizes (e.g., small, medium, large) of the delay elements of groups 501-503 enable a more consistent manner of increasing and decreasing the total amount of propagation delay of the delay signal path. For example, the delay elements of group 501 are each small delay elements which add a comparatively small incremental amount of delay to the delay signal path when an additional one of the delay elements is switched onto the delay signal path. The delay elements of group 502 (e.g. medium delay elements) each add a larger (e.g. medium) incremental amount of delay when switched onto the delay signal path. However, since the input signal has to propagate through all of the small delay elements of group 501, each additional medium delay element switched on to the delay signal path increases the total signal propagation delay by a substantially similar percentage (e.g., Δ% increase) as when a small delay element is switched on to the delay signal path.

The substantially constant Δ% increase/decrease relationship holds true for the large delay elements of group 503. Since the input signal has to propagate through all of the small delay elements of group 501 and all of the medium delay elements of group 502, each additional large delay element switched on to the delay signal path increases the total signal propagation delay by a substantially similar percentage (e.g., Δ% increase) as when a small delay element or a medium delay element is switched onto the signal path.

In this manner, embodiments of the present invention implement a tapered delay chain where the delay elements at the beginning of the delay chain are smaller with respect to the delay elements at the end of the delay chain.

In the present embodiment, the desired amount of delay is implemented via a configuration input 530 for the switch circuit 540. For example, depending upon the particular requirements of a given application, the configuration input 530 can set the switch circuit 540 to increase the delay amount, decrease the delay amount, keep the same delay amount, or the like. Although the three groups of different sized delay elements 501-503 are shown, it should be noted that a larger number of groups delay elements, and/or a larger number of delay elements per group, can be included within the tapered delay element chain 500 to give a larger range of adjustable delay and/or a greater degree of adjustment granularity. This is shown in FIG. 5 by the arrow 550. Similarly, fewer groups of delay elements (e.g., two), and/or fewer delay elements per group, can be used for those applications requiring a small range of adjustable delay in/or a lesser degree of adjustment granularity.

In one embodiment, for example, group 501 can include 36 small delay elements which provide 36 increments comprising the first 9% of the total available delay. The small delay elements would be comparatively fast circuits having a comparatively fewer number of devices per delay element. The 20th element of group 501 would typically have a 5% _ increase at the first element of the group and a 3% _ increase at the last element of the group. The medium size group 502 can include 36 medium delay elements comprising the second 16% of the total available delay, where the first element of the medium size group 502, similarly, would typically have a 5% _ increase and a 2% _ increase at the last element of the group. The large size group 503 can include 56 large delay elements comprising the last 75% of the total available delay, where the first element of the large size group 503 would typically have a 5% _ increase and a 1% _ increase at the last element of the group.

The adjustable delay capabilities of embodiments of the present invention can be advantageously used in a number of different applications. For example, in a high-performance memory application (e.g., DDR DRAMs) sampling windows correspond to the rising and falling edges of the strobe signals can be accurately placed at the center of the stringent rise-and-hold times. Additionally, for example, in high-speed signaling applications the rising and falling edges of multiple data signals can be accurately aligned with nanosecond accuracy (e.g., 1.875 nanoseconds for DDR II 533 DRAM).

FIG. 6 shows a diagram depicting the internal components of the switch circuit 540 in accordance with one embodiment of the present invention. As shown in FIG. 6, each of the taps 521-524 is activated or deactivated by its associated logic. The logic is set up via associated respective configuration bits stored within respective storage elements (e.g., flops) 611-614. In the switch circuit 540 embodiment, the first one of the storage elements 511-514 storing a logic zero results in its correspond tap being selected (e.g., from left to right). For example, in the switch circuit 540 embodiment, if the first through the “Nth” storage elements are 1 (e.g., from left to right), and the N+1 storage element is zero, the corresponding N+1 tap is selected.

In one embodiment, the storage elements 611-614 are accessed via their respective inputs 601-604. The inputs 601-604 thus comprise the configuration input 530 shown in FIG. 5, and can be accessed in parallel. In an alternative embodiment, the configuration bits can be shifted into the storage elements serially (e.g., from left to right from element 611 to 614).

It should be noted that the switch circuit 540 embodiment of FIG. 6 depicts a configuration where there is either one stacked inverter or an odd number of stacked inverters comprising each delay element. As described above, depending upon the number of stacked inverters per delay element, the signal emerging from one delay element to the next delay element will be inverted (e.g., for an odd number of inverter(s) per delay element) or un-inverted (for an even number of inverters per delay element). The logic of the switch circuit 540 embodiment is configured for an odd number of stacked inverters, whereby the inversion is properly handled by the depicted NOR gates. However, for example, for an even number of inverters per delay element, the NOR gates would be replaced by OR gates.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 

1. A method of delaying a signal comprising: selecting one of a plurality of delay elements using a switch circuit to create a delay signal path, wherein the plurality of delay elements comprise; a first group of stacked inverter delay elements each configured to implement a first delay by incorporating a first number of active devices per delay element; a second group of stacked inverter delay elements each configured to implement a second delay larger than the first delay by incorporating a second number of active devices per delay element, the second number larger than the first number; and a third group of stacked inverter delay elements each configured to implement a third delay larger than the second delay by incorporating a third number of active devices per delay element, the third number larger than the second number; coupling an input signal to a first delay element of the delay signal path using the switch circuit; propagating the input signal through the delay signal path; and receiving a resulting output signal from a last delay element of the delay signal path by using the switch circuit to generate a resulting output signal that is a delayed version of the input signal.
 2. The method of claim 1 wherein each of the plurality of delay elements is configured to add an incremental amount of delay to the input signal when switched into the delay signal path by the switch circuit.
 3. The method of claim 1 wherein the switch circuit is configured to adjust an amount of delay for the delay signal path by adjusting a number of the plurality of delay elements in the delay signal path.
 4. The method of claim 1 wherein the input signal propagates from a first delay element to a subsequent delay element using a substantially direct connection.
 5. The method of claim 1 wherein the switch circuit is configured to store a plurality of configuration bits to select a number of delay elements comprising the delay signal path.
 6. The method of claim 5 wherein an amount of delay added to the input signal is adjustable in accordance with setting the configuration bits in the switch circuit.
 7. The method of claim 1 wherein each of the delay elements comprises a stacked inverter delay element.
 8. A tapered chain of delay elements, comprising: a plurality of delay elements comprising: a first group of stacked inverter delay elements for implementing a first delay by incorporating a first number of active devices per delay element; a second group of stacked inverter delay elements for implementing a second delay larger than the first delay by incorporating a second number of active devices per delay element, the second number larger than the first number; and a third group of stacked inverter delay elements for implementing a third delay larger than the second delay by incorporating a third number of active devices per delay element, the third number larger than the second number; a switch circuit coupled to the plurality of delay elements and configured to select at least one of the plurality of delay elements to create a delay signal path having an amount of delay in accordance with a number of delay elements comprising the delay signal path; an input coupled to a first delay element of the delay signal path to receive an input signal; and an output coupled to the switch circuit, wherein the output is coupled to the delay signal path to receive a delayed version of the input signal after propagating through the delay signal path.
 9. The tapered chain of delay elements of claim 8, wherein each of the stacked inverter delay elements further comprises: a first plurality of devices of a first type coupled in series; a second plurality of devices of an opposite type coupled in series; and wherein said first and said second pluralities of devices are coupled in series.
 10. The tapered chain of delay elements of claim 9 wherein each of the plurality of stacked inverter delay elements are configured for a lower leakage current than an inverter comprising only two active devices.
 11. The tapered chain of delay elements of claim 9 wherein a gate of each of the plurality of stacked inverter delay elements is coupled to receive the input signal.
 12. The tapered chain of delay elements of claim 8 wherein the switch circuit is configured to adjust an amount of delay for the delay signal path by adjusting a number of the plurality of delay elements in the delay signal path.
 13. The tapered chain of delay elements of claim 8 wherein each of the plurality of delay elements is configured to add an incremental amount of delay to the input signal when switched into the delay signal path by the switch circuit.
 14. The tapered chain of delay elements of claim 8 wherein the input signal propagates from the first stacked inverter delay element to a subsequent stacked inverter delay element using a substantially direct connection.
 15. The tapered chain of delay elements of claim 8 wherein the switch circuit is configured to store a plurality of configuration bits to select a number of delay elements comprising the delay signal path.
 16. The tapered chain of delay elements of claim 15 wherein an amount of delay added to the input signal is adjustable in accordance with the setting of the configuration bits in the switch circuit.
 17. A tapered chain of delay elements, comprising: a plurality of delay elements comprising: a first group of stacked inverter delay elements each configured to implement a first delay by incorporating a first number of active devices per delay element; a second group of stacked inverter delay elements each configured to implement a second delay larger than the first delay by incorporating a second number of active devices per delay element, the second number larger than the first number; and a third group of stacked inverter delay elements each configured to implement a third delay larger than the second delay by incorporating a third number of active devices per delay element, the third number larger than the second number; a switch circuit coupled to each of the plurality of delay elements and configured to select at least one of the plurality of delay elements to create a delay signal path having an amount of delay in accordance with a number of delay elements comprising the delay signal path; an input coupled to a first delay element of the delay signal path to receive an input signal; and an output coupled to the switch circuit, wherein the output is coupled to the delay signal path to receive a delayed version of the input signal after propagating through the delay signal path.
 18. The tapered chain of delay elements of claim 17 wherein the switch circuit is configured to adjust an amount of delay for the delay signal path by adjusting a number of the plurality of delay elements in the delay signal path.
 19. The tapered chain of delay elements of claim 18 wherein each delay element comprising the delay signal path is configured to add the first delay, the second delay, or the third delay to the input signal in accordance with whether the each delay element is of the first group, the second group, or the third group of stacked inverter delay elements, respectively.
 20. The tapered chain of delay elements of claim 19 wherein the input signal propagates from the first stacked inverter delay element to a subsequent stacked inverter delay element using a substantially direct connection. 